1. Field of the Invention
The present invention generally relates to a method for producing a silicon substrate for solar cells.
2. Description Of Related Art
Solar cell is a very promising clean energy source, which can generate electricity directly from sunlights. However, the cost of the production of solar cells needs to be significantly reduced, so that the solar cells can be widely accepted as a major electricity source. Study shows that the silicon wafer share occupies over one third of the total cost of a crystal silicon (c-Si) solar cell module. Consequently, in order to reduce the cost, it has become an important development trend to make solar cells by multicrystalline silicon (mc-silicon) thin film.
Recently, all high-efficiency mono-crystalline silicon solar cells have a particular textured structure on the front surface, so as to reduce the reflection loss and to improve the light-trapping property. Currently, the most effective texturization method is to etch pyramids on mono-crystalline wafers with <100> surface orientation, in which method alkaline etching solutions of potassium hydroxide (KOH) or sodium hydroxide (NaOH) with anisotropic etching rates, where the etching rate of <111> planes are slower than that on <100> and <110> planes by two orders of magnitude are used. Therefore, randomly distributed pyramids are formed through intersecting <111> planes, which are called random pyramids, or inverted pyramids structure is formed by using a cross-hatched masking layer. Although the above method is an elegant and effective method for forming the surface texture of the mono-crystalline wafer, it is not suitable for the multicrystalline wafer, since the grains have different orientations. When the conventional alkaline etching technique is used, the random grain orientations of the multi-crystalline silicon wafer have an inhibited effect in forming the uniform and effective surface configuration. Therefore, the isotropic etching or other approaches that are not affected by the crystal orientation must be adopted to configure the above surface.
As for a wet chemical isotropic etching of multicrystalline silicon, hydrofluoric acid (HF) and nitric acid (HNO3) are appropriate, and such an etching process provides an undamaged smooth surface and achieves almost uniform reflectance distribution on the wafer surface. The reaction of these acids with silicon is strongly exothermic, so that phosphoric acid (H3PO4) or acetic acid (CH3COOH) may be added therein to control the etching process. However, the surface may become smoother and the reflectivity may be increased instead of decreased. Therefore, a surface-active agent is generally added into the etching solution to enhance the bubble-masking effect.
In addition, a wet chemical approach for texturing multicrystalline silicon has been disclosed in U.S. Pat. No. 6,790,785, where a nanometer-scale porous layer is etched on a multicrystalline silicon surface. A weighted reflectivity value obtained by this approach is reduced to 9%, but the short-circuit current density of the solar cell is significantly reduced. That's because the porous silicon etching may result in poor surface passivation and high absorption coefficient for short wavelength photons. Furthermore, as the direct band-gap behavior of the short wavelength response is reduced, the porous silicon layer has a appreciable absorption loss.
Furthermore, in some research, the silicon surface is textured by disposing first a metal-containing layer, for example, as disclosed in WO 2007/025536, the etching process being performed with a solution containing HF or NH4F, as well as an oxidant. However, it is quite complicated to control the thickness of the metal-containing layer in terms of a large surface area.